Lettuce And Fruit Salad With Poppy Seed Dressing

• 6 bibb lettuce heads
• 3 grapefruit
• 180 ml vinegar
• 180 ml oil
• 30 gr onion
• 30 ml vegetable oil
• 22 gr poppy seeds
• 14 gr sugar
• 5 ml dry mustard
• 3 gr salt